In fact since decoding also often happens in hardware, the raw size may never materialize anywhere in the pipeline, other than the framebuffer of the video card. Even HDMI has compression now [0]
The author probably choose a screenshot as a benchmark, because otherwise it's hard to get your hands on anything compressible but not already compressed.
Throw in YUV compression and you can shift 120hz 8k with relatively little problem.
But there are clearly other ways to store a pixel of colour information in less than 3 bytes, which is OP's point. It's not an optimization really - it's just a different coding format (Just as ASCII isn't an optimization of Unicode).